The expression "a very good color rendering" in the present description and claims is understood to mean that the general color rendering index R(a,8) (mean of the color rendering indices of eight test colours as defined by the Commission Internatinale d'Eclairage: IEC Publication No. 13.2 (TC-3.2), 1974) has a value of at least 90.
The color of visible radiation is characterized by the color coordinates (x, y) which define the color point in the color triangle (see IEC Publication No. 15 (E-1.3.1), 1971). Low-pressure mercury discharge lamps for general lighting purposes should emit light which can be designated as "white". White radiation is found in the color triangle at color points situated on the Planckian locus. This locus, also called black body line and designated hereinafter as curve P, contains the color points of the radiation emitted by a perfect blackbody at different temperatures (the so-called color temperature). A given color temperature is assigned not only to a given point on curve P, but also to radiation having color coordinates situated on a line which intersects the curve P in that point (see the said IEC Publication No. 15). If this radiation has a color point close to the curve P, this radiation is also regarded as white light with this given color temperature. In the present description and claims, the expression "a color point close to the curve P" is understood to mean that the distance from the color point to the point on the curve P with the same color temperature is at most 20 MPCD. MPCD (minimum perceptible color difference) is the unit of color difference, see the publication by J. J. Rennilson in Optical Spectra, October 1980, p. 63. In addition to requirements to be imposed on the luminescent layer regarding the color temperature of the light radiated by the low-pressure mercury discharge lamp and the color rendering, it is also necessary for the luminescent layer to comprise substances having a small short-term luminous decrement (i.e., the luminescent materials suffer no or only little damage from irradiation with short-wave ultraviolet radiation, especially 185 nm radiation).
Low-pressure mercury discharge lamps which radiate white light and have a very good color rendering are known from European Patents EP-A-0114441 and EP-A-0124175. Calcium halophosphate activated by antimony and/or manganese is often used as the third luminescent material. It is found in practice that the requirements as to the color of the light radiated by the low-pressure mercury discharge lamp and as to color rendering can only be complied with in the known low-pressure mercury discharge lamp in combination with a comparatively low luminous efficacy of the low-pressure mercury discharge lamp.